How do you transport 25 tons of gold across land and sea?

My guess is air freight is the safest method, does it matter if it goes on a DHL 777 or a Commercial 777? Either way, the plane could be hijacked.

But I suspect you'd have somebody with an insurance claim for $1 billion in gold, and an insurance company going bankrupt as a result.

It does happen, security by obscurity is actually quite prevalent outside of Infosec: if nobody knows it's there how can it be stolen?

Search for "Heathrow Runway Robberies", etc. There were a number of documentaries on the subject, IIRC there were several gangs that specialized in jacking crap off planes in airports. It's possible (however unlikely) that some graduated beyond the airport.